Following the National Ignition Facility’s first positive-gain ignition shot, Livermore researchers are reaching higher energy yields from existing tools while looking ahead to new opportunities.
In Lawrence Livermore’s storied history of achievement, the Laboratory’s first successful positive-gain ignition shot at the National Ignition Facility (NIF) on December 5, 2022, ranks near the top of the list. Achieving ignition is one of the most significant scientific advances of our time, enhancing our understanding of physical and materials science; supporting the Stockpile Stewardship Program to ensure the safety, security, and reliability of the nation’s nuclear weapons stockpile; advancing new technological innovations; and making the long-hoped-for goal of fusion energy closer to reality than ever before.
Livermore began pursuing ignition in the 1960s and spent the interim decades synthesizing theory, laser technology, instrumentation, computation, engineering, modeling, materials development, fabrication, systems integration, and data analysis to make ignition happen. When the laser was invented in 1960, several Livermore scientists, including John Nuckolls (who went on to become Laboratory director), sought an energy source to initiate a small thermonuclear explosion. In 1962, physicist Ray Kidder convinced then Livermore Director Johnny Foster that a scaled-up laser could achieve fusion, and he began to lead the Q project to fulfill this objective. Kidder also developed computational modeling capabilities for inertial confinement fusion (ICF) that were a precursor to Lasnex, currently in use at NIF. (See the highlight Cracking the Fusion Code(s).)
At Lawrence Livermore, stockpile stewardship demands increasingly better understanding of the fundamental physics behind nuclear fusion and high-energy-density (HED) conditions, which then informs Laboratory researchers on the behavior of both nuclear and nonnuclear materials under extreme pressures and temperatures. Prior to the 1996 Comprehensive Nuclear-Test-Ban Treaty, underground nuclear tests provided this data. In the absence of nuclear testing, NIF’s laser platform was tasked with replicating the conditions of nuclear detonation safely in a controlled laboratory environment. Now, shot data input into sophisticated computational models predict a range of potential outcomes and material reactions. Furthermore, NIF experiments provide data for assuring that decades of component aging would not affect the viability of the nation’s nuclear weapons. Increasing fusion yield—by increasing the fraction of deuterium–tritium (DT) fuel burned (see the box below, "The Process of Ignition")—creates greater opportunities to refine the models that predict weapon performance by providing more accurate data related to nuclear detonation conditions.
The Process of Ignition
Every ignition experiment begins with NIF sending a weak laser pulse along a series of optical fibers to 48 preamplifiers, which increase the pulse energy from 1 billionth of a joule to several joules. Next, these preamplifiers split the laser into four beams that are then injected into NIF’s 192 main laser amplifier beamlines, which further increase the power of the beams. After only a few millionths of a second, the total energy has increased to 4 megajoules (MJ).
The 192 beams subsequently travel to two switchyards on both sides of NIF’s Target Chamber where they then split into quads of two-by-two arrays. The quads pass through a final optics assembly that converts the pulses from 4 MJ infrared to 2 MJ ultraviolet energy, all of which is focused on the target in the center of the Target Chamber. The target is a cylindrical hohlraum that houses a tiny capsule holding deuterium (D) and tritium (T), two forms of hydrogen. NIF’s lasers heat the hohlraum to temperatures more than 3 million degrees Celsius, which generates x-rays that blow off the ablator (the target capsule’s surface). Such high temperatures cause an implosion that compresses and heats the DT fuel to extreme temperatures and densities, which then causes the hydrogen atoms to fuse, create alpha particles (helium nuclei), and release high-energy neutrons and other forms of energy. In successful ignition shots, the alpha particles spread and heat the surrounding cold fuel, which triggers a self-sustaining fusion reaction.
Perhaps the overarching question facing NIF and Lawrence Livermore today is, “What now?” Building on the lessons learned from earlier NIF experiments and ignition shots, the Laboratory’s ICF team has continued to push the boundaries of what can be accomplished given the facility’s current parameters. Much more can be learned in support of stockpile stewardship and other national security missions. While the promise of fusion energy is closer than before, practical implementation of this method is still years away. (See the highlight From Ignition to Energy.) Looking ahead, Livermore continues to conduct ignition shots, and work is underway, through the Enhanced Yield Capability (EYC) project, to increase ignition yields as much as possible given the current tools and technology at hand and the constraints those factors pose. Annie Kritcher, ICF program team lead and lead designer of the 2022 ignition shot, is looking ahead to what can be accomplished at NIF under the EYC project and in the future. She says, “The EYC project will help us begin to answer questions about what we’ll need for a next-generation facility, both for HED science and inertial fusion energy (IFE).”
Long Journey to Ignition
The Laboratory’s Laser Program began pursuing ICF with a proposal to the Atomic Energy Commission to construct a 10-kilojoule (kJ) laser later in the decade. (See S&TR, April/May 2022, Beaming with Excellence.) Starting with the 10-joule (J) Janus laser in 1974, which enabled Livermore to demonstrate a controlled thermonuclear reaction in laser-imploded DT fuel capsules, researchers could establish critical diagnostic techniques and improve their understanding of laser–target interactions. From this foundation, Livermore built the two-beam Argus laser in 1976, the first with spatial filters to relay the beam from one amplifier to another while preventing amplification spikes that cause filamentation—a type of optical damage.
In 1977, Livermore delivered 10.2 kJ of infrared laser energy on the first full-power firing of the newly completed 20-beam Shiva laser. Experiments on Shiva were important in showing how the wavelength of infrared laser light affects the implosions. “Our implosions were not performing well, but we had a good idea that the laser was interacting with the plasma and making hot electrons, which penetrate the implosions prematurely, preheat the target, and prevent a good implosion,” says Mordy Rosen, a long-time Laboratory physicist. Rosen started at Livermore in 1976 and designed the targets for the first Shiva experiments, working closely with computational experts to deepen the Laboratory’s understanding of the fundamental physics behind their work. “The Laboratory’s computational capabilities were so extraordinary, I could perform numerical experiments without waiting for the actual experiment to be completed. Even when the actual experiments didn’t perform as well as we would have liked, that was just the icing on the cake because now we had juicy mysteries to solve,” says Rosen.
By 1986, the Laboratory had deployed the Nova laser, which was more than 10 times more powerful than Shiva and could produce up to 150 kJ of infrared and 40 kJ of ultraviolet laser light in 2.5-nanosecond pulses, reaching up to 16 terawatts (TW) of power. Nova offered opportunities to run more advanced experiments that informed the ongoing planning of much larger lasers that evolved into the design of NIF. In doing so, the Nova laser played a role in the evolution of national deterrence. Rosen explains, “Nuclear testing ended with the end of the Cold War, but we couldn’t also afford to lose expertise in the field of high-energy-density physics. Maintining this expertise was a crucial consideration in earning the go-ahead for NIF’s construction and use. We needed the data NIF would generate and also felt that, in this new world, deterrence would be bolstered through achievement of ignition.”
In 1997, Livermore broke ground on NIF, the new facility for the 192-beam, 1.8-megajoule (MJ), 500-TW laser that would ultimately achieve ignition. Nuckolls, Rosen, and others played a key role in shaping and informing NIF’s work toward ignition, including an advisory role in “red teaming,” pointing out key problems that the researchers would need to solve. Part of this work involved improving hohlraum efficiency by changing the material of the walls, which increased the amount of energy (drive) that could be delivered to the target and achieve ignition.
Campaigning for Success
Just as the road to ignition started long before the December 2022 achievement, each NIF measurement campaign has a long lead-in. Measurement campaigns begin with a series of proposals submitted to a Technical Review Committee at least eight months to a year and a half in advance. Each proposal focuses on specific research aims and includes multiple shots using NIF’s lasers. Proposed experiments need not always be ignition shots; many experiments are conducted to explore fundamental physics questions or understand weapon survivability, for example.
In each case, a combination of factors that can be measured directly or indirectly from the diagnostics available is analyzed. (See the highlight Advanced Diagnostics Reveal Fusion Physics.) Keyhole shots to test the ablator and early-time laser delivery, symmetry experiments to shape the implosion, and other tuning shots often proceed layered implosion experiments. One or more of these shots may be needed to demonstrate the capabilities of a particular campaign. With knowledge gained from the right set of integrated experiments, Livermore physicists make incremental improvements to the performance metrics associated with ignition. Integrated experiments will have many useful data, but the most widely understood for ignition purposes is neutron yield.
When a project proposal is accepted, Livermore project engineers are involved from the earliest phases with NIF physicists to identify potential outcomes as well as key unknowns. Brandon Woodworth, the ICF program chief engineer, compares an integrated layered experiment to automobile racing. “On race day, the driver wants to have confidence that the race car will perform well,” he says. “In that context, we might do focused physics experiments at NIF that are equivalent to studying how different tire treads impact the temperature of the car’s wheels, or the impact of alternative fuels on performance to demonstrate some benefit.” Building on tuning shots, the team might move on to more integrated experiments involving a high-energy 2.2 MJ drive layered implosion for the platform to demonstrate “race-day” performance.
After each experiment, the lead researcher and project engineering team work together to resolve any problems or questions, such as small variations on the laser pulse or diagnostic setup adjustments. Along with the target design and fabrication teams, they establish definitions for future targets or adjust individual target parameters such as the capsule thickness. Once the data is collected, post-shot analyis begins, including high-performance computing (HPC) simulations to inform how the experimental results aligned with predictions. These simulations then inform the next series of shots.
In the first few hours after a layered experiment, available data remains limited. Yield numbers are ready soon after, as is the backscatter data, which measures the intensity and characteristics of light scattered toward the laser source. Backscatter data helps the team understand if something went wrong with the laser–plasma interaction. More detailed results across the experiment’s parameters come a few weeks later. For example, radiographs—x-ray images captured on film—must be developed and processed.
After the data is collected, post-shot HPC simulations recreate implosion conditions and identify which factors had the most meaningful effects. Woodworth says, “We have a variety of robust and widely used simulation tools to model our experiments, and understanding what experiments we can do to constrain parameter space for those simulations is helpful to better predict the outcomes we’re aiming for.” Kritcher adds, “Over a period of months, sometimes years, we develop a deep understanding of what occurred in that experiment and how that motivates changes for future experiments.”
Making adjustments based on experimental data can vary in complexity. For example, adjusting the density of the gas in the hohlraum is as simple as turning a knob or changing a setting. But altering physical attributes of the hohlraum itself can take several months and requires a team of engineers, physicists, and fabrication experts to identify and articulate the changes and then manufacture a new hohlraum. Such changes are meticulously tracked in datasheets that identify the campaign under which the experiments are run, the platform being tested (keyhole, symmetry, DT fuel), and other parameters.
Woodworth explains that a particular campaign will often use one hardware design while changing other parameters such as laser delivery or features of the target capsule. For example, the Hybrid-B campaign conducted from 2017 to 2018 studied high-density carbon (HDC, diamond) capsule size, grain structure, gas fill density, and laser entrance hole size in a common platform. The HDC ablator showed good performance, but the hohlraum was designed for 2.1 MJ laser energy and was too big to efficiently provide energy to the capsule with the 1.9 MJ that could be achieved at that time. In 2019, the Hybrid-E campaign, which included the first ignition shot, differed from the Hybrid-B campaign by using crossbeam energy transfer to improve symmetry and a smaller 6.4-millimeter (mm)-diameter hohlraum compared to the Hybrid-E campaign’s 6.72-mm-diameter hohlraum, among other design adjustments, to increase radiation drive on the capsule. By August 2021, the target and laser parameters impacting symmetry and drive in Hybrid-E campaign experiments had been adjusted following initial tests. Both reducing the size of the capsule by 50 micrometers and reducing the diameter of the laser entrance holes from 3.64 mm to 3.1 mm enabled coupling more of the laser energy to the capsule and scientific ignition (measured experimental values that the plasma achieved certain temperature, density, and confinement time) on August 8, 2021. Subsequent design changes, including increasing the thickness of the capsule, further increased the fusion energy produced.
Targeting the Future
NIF has successfully achieved ignition—exceeding unity gain, or breakeven (when input energy is equal to output energy)—eight times as of late May 2025. Subsequent experiments have involved many adjustments and improvements to increase yield, resulting in an increase in output gains from 1.5 to greater than 4. NIF’s April 7, 2025, experiment used 2.08 MJ of laser energy to deliver an authorized yield of 8.6 MJ to the target, producing a target gain of 4.13. These gains are due in part to a threshold effect, in which marginal increases in target quality and energy delivery can have outsized effects on burnup fractions and yield.
The ICF program team has gathered many new insights into bigger yields and more predictable outcomes, although questions remain. A key factor in the continued instances of ignition has been improvements to the target capsule. Leading up to the December 2022 shot, the team had identified that the target it would use had many high-Z (high-density particles identified using x-rays) inclusions, which they suspected would impact the shot’s performance. In a repeat attempt, the team tested a higher quality capsule and saw a higher performance outcome, NIF’s second ignition shot on July 30, 2023, (2.05 MJ laser power, 3.88 MJ yield). Kritcher says, “Since the initial ignition experiment, we’ve learned more about the impact of target quality in our designs. We’ve also learned more about the implosion symmetry of these designs and how the shape changes in time.” Understanding and predicting implosion symmetry has been a significant effort at NIF, and the teams have developed future efforts to address the issue in the coming year. “From the viewpoint of certain experimental diagnostics (imaging of the hot ignited core), the implosion might appear symmetric, but that doesn’t necessarily mean it’s sufficient. We’re learning more about the DT shell symmetry that surrounds the hotspot and are devoting a lot of effort to making that shell more uniform,” says Kritcher.
Several experiments have revolved around improving target capsule quality, including altering the dopant distribution in the shell. The HDC shells are doped with tungsten to control the hard x-ray preheat in the implosion. Rather than using a buried-layer dopant distribution, which introduces dopant atoms into the shell material just below the surface, the team used a ramped distribution, in which the dopant concentration changes over 6 micrometers, ramping up and down smoothly. “We also paid attention to the uniformity of the dopant on the HDC shells,” says Dan Clark, the deputy program lead for modeling in the ICF program. “When target fabrication coats the shells in a pulse mode, the shells rattle around in a pan as they’re being coated, which can result in significant fluctuations in the amount of dopant rather than a uniform layer. Any discontinuities in the shell will then have a negative impact on the shot.” Following these material improvements, the next shot saw a 30 percent increase in yield from the repeat of the December 5, 2022, experiment, indicating that the mix seeded at the dopant boundary can be a substantial degradation source and can illuminate past performances.
Dopant distribution testing has not been limited to experiments; researchers have also modeled the dopant mix in computational simulations. HPC simulations are crucial to ensuring accuracy down to the tiniest details. “We’re working with challenging conditions, such as very short time and length scales in which everything happens both very quickly and in a very small space,” says Clark. “The modeling showed there would be a substantial mix effect in the implosions. For years, we’ve been questioning how changing the target might mitigate this mix effect. We iterated with target fabrication over years to produce a target that met our specifications for that particular experiment.” Clark predicts that NIF will adopt this dopant mix and distribution strategy for future experiments and campaigns. “From the design side, we’re also looking at other ways to smooth the doping out even more to improve the result further,” explains Clark.
In addition to dopant distribution and mix, researchers have been adjusting the thickness of the target capsule’s shell as well as pursuing different ablator materials. Clark explains, “To an outsider, the change is almost unnoticeable, on the order of micrometers in shell thickness. While small compared to the overall dimensions of the laser system, this kind of change has a significant impact on symmetry and performance.” For ablator materials, the team is considering boron carbide, an intermediate between the plastic material used for ablators in the past and the HDC in use.
Enhancing Current Capacity
While difficult to unequivocally state that any one part of NIF is more important than any other part, the 192 lasers are undoubtedly crucial to achieving ignition. Given the laser power and energy available now at NIF, yields in the 100 MJ range are not possible, but many ways to increase yields still exist before construction of a new facility begins. The current burnup fraction is approaching 12 percent; however, significantly increasing burnup remains a challenge under NIF’s current capabilities. “We must stay at the same laser power because of potential issues with the damage threshold of laser optics. The lasers weren’t originally meant to operate at these high energies and powers,” says Kritcher. As a result, the laser team has steadily worked on methods to eke out more laser energy from the system and ways to couple this energy to the implosion, such as improvements in optics reprocessing that will enable higher energy to hit the slabs of glass in NIF without damage. “We have several ways we might couple this extra laser energy to the implosion, even at fixed power,” she continues. “Possibilities include making the ablator thicker, driving the same implosion with reduced coasting time, keeping the laser on and letting it ‘cook’ longer, and improving the conversion of this laser energy to ‘drive.’ We’re exploring all of these options.”
The EYC project is Livermore’s interim step to increasing yield while planning ahead for a bigger facility in the future with more capabilities toward sustained ignition and burn. The U.S. Department of Energy (DOE) approved a Critical Decision-0 (CD-0) in September 2024 to formally establish the EYC project and begin the process of conceptual planning and design. The ultimate goal of the EYC project is to develop, design, and implement upgrades to NIF, such as installing laser glass in open ports in NIF’s power amplifiers to increase the facility’s laser energy from 2.2 MJ to 2.6 MJ, potentially enabling fusion yields as high as 30 MJ.
Kritcher explains, “We’ve been tasked with developing a set of designs that will work with an enhanced NIF while still operating within similar laser power constraints. We anticipate many iterations on these designs, but a key focus was to leverage our experience from NIF to predict a range of fusion yields we could expect with 2.6 to 3 megajoules of energy, and how studying these implosions could help us progress toward the next-generation facility.” Early designs considered higher laser powers—approximately 500 to 600 TW and 2.6 to 3.0 MJ. The approved designs were constrained to be limited in power and energy (given uncertainty in laser damage thresholds) to 450 TW and 2.6 MJ, which would pertain to the HDC ignition platform already in use.
With a CD-0 approval in place, the EYC project is only at the beginning stages of planning, with much more coordination with DOE and the National Nuclear Security Administration to move from planning to action. In the meantime, Livermore is implementing the NIF Sustainment Plan to preserve existing function. (See S&TR, March 2025, Sustaining NIF's Bright Future.) Even before the start of EYC project implementation, the next-generation facility is also top of mind. Clark says, “We’re thinking about much higher laser energies and powers, which opens up a new world of possibilities. However, we can only capitalize on that world if we improve our understanding of what we’re doing right now.”
NIF’s successes have been broadcast far and wide, with the December 2022 ignition shot garnering global attention. This attention has also attracted further interest in IFE, a development that poses exciting new opportunities for Livermore. “I’m really excited about how our work at NIF is energizing the IFE community,” says Kritcher. “I joined the Laboratory because of my passion for IFE, and it’s incredibly exciting to see the momentum that these experiments are generating.”
The potential for fusion energy provides excitement to new generations as well. In his long tenure at the Laboratory, Rosen has had the opportunity to see the development of Livermore’s laser research efforts, as well as the impact the work has had on the broader community. “Way back when I started, I was hoping this would be a fusion energy source, and I still want that,” says Rosen. “Now younger generations are realizing it’s a genuine possibility. When my kids heard about the ignition achievement and the possibility of making a fusion energy future, they were genuinely moved. What moves me most is the reaction of their children, my grandkids, that these outcomes have given them hope for the future.”
—Sheridan Hyland
For further information contact Annie Kritcher (925) 423-6919 (kritcher2 [at] llnl.gov (kritcher2[at]llnl[dot]gov)) or Dan Clark (925) 423-8759 (clark90 [at] llnl.gov (clark90[at]llnl[dot]gov)).
